Motor drive circuits that are designed for use in automotive applications must be highly reliable and be able to withstand "reverse-battery" conditions without failing. A "reverse-battery" condition occurs when the terminals of an automotive battery are connected with the wrong polarity.
A conventional automotive motor drive circuit 10 is shown in FIG. 1. The illustrated circuit includes a motor 12 that is powered by a battery 14 which is part of an automotive charging system. Connected in parallel with the battery is a diode 16 and a relay coil 18. With the battery connected to the coil 18 as shown, the coil is energized to close a contact set 20 and thereby to couple a circulating diode 22 in parallel with the motor 12.
An FET (field effect transistor) 24 is connected between the motor and the negative terminal of the battery and, as shown, the FET has an "intrinsic diode" 26 coupled between its drain and source. The term "intrinsic diode" means a diode which is inherently part of the semiconductor structure which forms the FET. To turn the motor on, a control pulse P is applied to the gate of the FET 24. When the control pulse is removed, recirculating current flows through the diode 22 and the motor 12.
The arrangement of the relay coil 18, contact set 20 and diode 16 prevents a high level of current from flowing when a reverse-battery condition occurs. In that situation, the diode 16 becomes reverse biased, thereby de-energying the coil 18 and opening the contact set 20. As a result, no current flows through the diode 22. Although current does flow through the diode 26 and the motor 12, the resistance of the motor winding limits the current to a safe level.
It is desirable to avoid the use of relays in the type of application discussed above, not only for cost reasons, but also to provide a more reliable drive circuit for motors and other types of inductive loads.